Experimental measurement and modelling of heat transfer in spiral and curved channels

Ghobadi, Mehdi (2014) Experimental measurement and modelling of heat transfer in spiral and curved channels. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Heat transfer enhancement is desired in most thermal applications. In general, there are two methods to improve the heat transfer rate: active and passive techniques Active techniques are based on external forces such as electro-osmosis, magnetic stirring, etc. to perform the augmentation. Active techniques are effective; however, they are not always easy to implement with other components in a system. They also increase the total cost of the system manufacturing. On the other hand, passive techniques employ fluid additives or special surface geometry. Using the surface geometry approach is easier, cheaper and does not interfere with other components in the system. Surface modification or additional devices incorporated in the stream are two passive augmentation techniques. With these techniques, the existing boundary layer is disturbed and the heat transfer performance is improved. However, pressure drop is also increased. Curved geometry is one of the passive heat transfer enhancement methods that fit several heat transfer applications such as: compact heat exchangers, steam boilers, gas turbine blades, electronics cooling, refrigeration and etc. This dissertation contains eight chapters.. Chapter one is the introduction and shows the originality, novelty and importance of the work. Chapter two reviews the literatures on the heat transfer and the pressure drop correlations in curved circular tubes. In chapters three and four, two heat sinks having spiral and straight channel geometry engraved on them are examined experimentally. Heat transfer and pressure drop inside them are measured, and reduced to apply two existing correlations to predict their behaviour analytically. In chapters five, six and seven, thermal and flow behaviour inside curved geometry are studied experimentally. The calculated heat transfer coefficient and pressure drop are compared to the existing models. Comparing the predicted Nusselt number from the existing models, poor accuracy was observed in the region of 5 < Pr < 15. Finally, in chapters six and seven two new asymptotic correlations are proposed to calculate the heat transfer and the pressure drop inside mini scale curved and coiled tubes.

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